Heads Up Displays in Commercial Airliners
Head up displays allow airline pilots to keep their focus outside the aircraft during takeoff and landing. The HUD provides the most important navigation and performance data needed during those flight phases, data like pitch, heading, airspeed, and altitude. Some HUD systems also provide virtual imagery for use during low visibility ground operations.
A HUD is a type of collimated display. Collimation places the HUD imagery at “optical infinity”, a somewhat useless term IMHO because it’s hard to relate to. In any case, what’s important is that there are no perspective shifts of the HUD imagery relative to the outside view when the pilot moves his head. During installation, the HUD is very carefully aligned with the aircraft structure (“boresighted”) so that HUD imagery is aligned with the outside view. Bottom line: the collimated view of the HUD virtual horizon line appears on top of the real horizon and stays there regardless of changes in view point.
An airline HUD consists of three major components: the processor which takes data from the aircraft then generates the HUD video data, the projection unit which is located above and behind the pilot, and the collimator/combiner which is located directly in the pilot’s line of sight during use and folded up against the ceiling when not in use.
The collimator/combiner is a spherical-section partially-reflective mirror. It transmits about 80% of the light from the outside view. Remember, the goal is to allow the pilot to keep his attention outside the aircraft. He can’t do that if the HUD blocks his view. The spherical shape of the collimator/combiner collimates the HUD image, but also significantly distorts it. The projection unit lens is designed specifically to correct this distortion so the HUD image will correctly overlay the outside view.
The HUD image fills a roughly 30 degrees horizontal by 25 degrees vertical field of view. The image is normally monochromatic green. A “color HUD” is capable of displaying a second, warning color to call attention to critical items.
Flight sim enthusiasts often assume that a functioning HUD is out of reach. While it’s true that an airline HUD is complex and expensive, what’s often missed is that the key design goal of an airline HUD is different than that of a HUD for use in a simulator. The airline HUD must place an undistorted image on top of and in alignment with the real outside view located at optical infinity. A simulator HUD should place an undistorted image on top of and in alignment with a simulated outside view which may be all of six feet away on a projection screen.
A flight-rated HUD requires collimation and distortion correction. A simulator HUD requires neither.
Assume now, for the sake of brevity, that simulator HUD imagery is available on a separate monitor.
An easy way to combine what’s on the HUD monitor with the simulator out-the-window scenery is to view the HUD image as a reflection in a partially transparent mirror and the out-the-window scenery through that same mirror. If the optical path length from the view point to the monitor is the same as the path length from the view point to the out-the-window image, the two images will be coincident and there will be no perspective movement between them when the view point shifts. Since the mirror is flat, there is no image distortion and no need for corrective optics.
A limitation of this approach is that will you need a big monitor to gain a wide HUD field of view. If you’re willing to accept a somewhat smaller field of view, and the out-the-window image is closer rather than farther, you can add realistic airline HUD functionality to your sim.
A source of glass and plastic partially-transparent mirrors is Telepromptermirrors.com.
Mike Powell, author of
Building Recreational Flight Simulators ,
Building Simulated Aircraft Instrumentation, and
Building Simulator Displays Systems. (A work in progress)